EPR imaging (EPRI) of oxygen provides a unique combination ot spatial resolution, oxygen resolution, and uniform sensitivity with depth in tissue, EPR is sensit,ive to temperature, free radicals, viscosity, pH and a number of other aspects of tissue fluids, which can be imaged. This Center focuses on the development of optimized electron paramagnetic resonance imaging (EPRI) technology to directly image these important aspects of in vivo physiology. This consortium between the Universities of Chicago, Maryland and Denver, is built on the recognition that successful EPRI requires coordinated development of optimum imaging instrumentation, synthesis of spin probes and spin traps to sample and report the tissue fluid environment and optimizing imaging strategies to sample and analyze the images. High resolution imaging of oxygen in tumors and normal tissues of living animals will continue to be the dominant focus of the Center. In the previous funding period the Center has obtained mages of oxygen in mouse tumors with the high spatial and oxygen resolution initially proposed. Novel trityl spin probes with extremely narrow spectral lines, have been synthesized to further improve both spatial and oxygen resolution of EPR images. Larger EPRI instruments have been built, and images of larger samples with high oxygen resolution have been obtained. Three specific aims focus on development and optimization of enabling technologies for improved oxygen imaging. Continuous wave (CW) acquisition techniques will be optimized, and pulsed acquisition techniques will provide higher signal-to-noise and reduced image acquisition times. Newer spin probes with narrower spectral lines will further improve sensitivity and extend the applicability of the new CW and putsed acquisition technologies. Acquisition and analysis strategy improvements will reduce imaging time, crucial for physiologically relevant imaging. This will permit oxygen imaging with higher resolution, and imaging of much larger animals. The development of technology to detect and image oxygen centered and nitric oxide free radicals in living animals with novel spin traps for these free radicals is a secondary physiologic goal. Temperature imaging will provide a specific focus within this area of wide spectral line imaging technology.